1. Field of the Invention
This invention relates to chemical exchange between hydrogenous compounds, in particular to deuterium exchange between hydrofluorocarbons and amines.
2. Description of the Prior Art
As used in this specification and the appended claims, terms such as "hydrogen," "water" and "hydrofluorocarbons" designate materials which include the naturally occurring abundance of deuterium, which is generally in the range from about 0.010 to about 0.016 mole percent relative to the hydrogen content. Substantially pure (isotopically) materials are referred to in symbolic form; e.g. H.sub.2 O and D.sub.2 O. The term "deuterium-enriched" means material whose deuterium concentration has been increased by an exchange process to a level greater than that before the exchange, regardless of whether or not the enriched material has a deuterium concentration greater than the natural abundance. Conversely, "deuterium-depleted" material has had its deuterium concentration reduced by an exchange process or other process to a concentration which may or may not be lower than the natural abundance. Also, as used in this specification and the appended claims, the term "chemical reaction" is meant to include unimolecular reactions, such as isomerization, dissociation and predissociation, in addition to more conventional chemical reactions.
Heavy water (D.sub.2 O) finds important use as a neutron moderator and coolant in certain atomic power reactors. These reactors are typified by the CANDU (Canadian deuterium-uranium) reactor. They have the important advantage that they do not require the use of uranium enriched in U.sup.235 as a fuel; instead, they use lower-cost natural uranium. They do, however, require large quantities of heavy water. Typically, 850 kg of D.sub.2 O are required per MW of installed electrical capacity. The D.sub.2 O cost constitutes about 15% of the capital costs for the power plant. With growing worldwide interest in heavy water reactors, some estimates project D.sub.2 O requirements for the atomic power industry to reach 9000 Mg per year by the turn of the century. Accordingly, low cost schemes for producing D.sub.2 O are required.
At present, large-scale production of heavy water is based on the Girdler sulfide (GS) process, which involves an isotopic exchange process between hydrogen sulfide (H.sub.2 S) and water. The overall exchange mechanism can be described by the dual temperature reaction: ##STR1##
Using this technology, an enrichment factor, .beta., defined as the ratio of the concentration of deuterium in the product to its concentration in the feed material, an effective value of 1.3 is obtained in a single elemental stage of enrichment. After multiple enrichment stages, the final deuterium content in the water using this exchange technique is on the order of 20%. Further deuterium enrichment to the required 99.75 atom percent D is conveniently done by water distillation. The requirement of hundreds of separative elements, coupled with the low D/H ratio in natural water (.about.1.5.times.10.sup.-4), make it necessary to process nearly 40 000 moles of feed material for each mole of product. This makes GS heavy water plants both highly capital intensive and highly energy intensive.
In addition to the GS process, three other chemical exchange processes have been commercialized; namely, water-hydrogen, ammonia-hydrogen, and methylamine-hydrogen. Although each of these processes requires less energy and has a higher single-stage deuterium enrichment factory than the GS process, the latter process is preferred for large-scale operations. The distillation of water and of hydrogen have also been used commercially for heavy water production, but only the latter is economically competitive with the GS process, and then only for small-scale operations. A number of other processes for the production of heavy water have been investigated. These include water electrolysis, combined electrolysis and catalytic exchange, hydrogen adsorption on palladium, methane-hydrogen exchange, hydrogen diffusion, and water crystallization. At present, none of these processes appears to be economically competitive with the GS process.
Laser isotope separation (LIS) provides the basis for additional techniques for D.sub.2 O production. In some cases, LIS techniques provide much higher single-stage enrichment factors than does the GS process. Marling and Herman reported the dissociation of CF.sub.3 CHCl.sub.2 (refrigerant 123) using infrared radiation from a CO.sub.2 laser with 1400-fold enrichment of deuterium concentration in a single step (Appl. Phys. Lett. 34, 439 (1979)). However, this process was later found to be impractical for commercialization. In particular, the infrared absorption ratio for deuterium vs hydrogen containing molecules is only about 100:1. Hence, when natural CF.sub.3 CHCl.sub.2 is irradiated, about 98% of the laser energy is absorbed by hydrogen-containing molecules and converted to heat.
Another LIS process that is useful as a step in D.sub.2 O production involves isotope-selective dissociation of gaseous trifluoromethane (TFM). Natural TFM, containing about 0.015% CF.sub.3 D, is irradiated with a CO.sub.2 laser at an appropriate infrared frequency (for example, 975.+-.50 cm.sup.-1). The CF.sub.3 D absorbs the radiation and dissociates according to EQU CF.sub.3 D+nh.nu..fwdarw.:CF.sub.2 +DF
followed by EQU 2:CF.sub.2 .fwdarw.C.sub.2 F.sub.4.
CF.sub.3 H absorbs negligible radiation and undergoes little dissociation. The DF product is separated from the remaining gas by conventional chemical methods, such as reaction with a metal oxide. Single-step enrichment factors of about 10.sup.4 have been demonstrated with this process.
There are several advantages of the TFM deuterium enrichment process over the currently used commercial processes. The extremely high enrichment factor allows a considerable reduction in the volumes of materials being handled in a separation plant at one time, which greatly reduces capital requirements and energy consumption. In the LIS process, nearly all the deuterium can be stripped from the feed material, but in the GS process, thermodynamic restrictions limit the deuterium extraction to 21%. Furthermore, in the GS process, the H.sub.2 S working medium is highly toxic, whereas TFM is essentially non-toxic. In the methylamine-hydrogen exchange process, the thermodynamic limitation on D.sub.2 extraction is 55%.
Additional details of the process for isotope separation by selective dissociation of TFM with an infrared laser are provided in copending U.S. application Ser. No. 25 978, filed Apr. 2, 1979. The disclosure of said copending application is incorporated herein by reference. To use the process therein disclosed in a commercial heavy water production process, it is essential that the TFM be continuously recycled and its deuterium content replenished through chemical exchange with a protolytic solvent. In view of the low naturally occurring D/H ratio, the deuterium for replenishment must ultimately be derived from an inexpensive feedstock available in large quantities, such as water, hydrogen or ammonia. This requirement does not restrict the exchange medium used to replenish TFM to one of these materials, however, since an exchange medium can, in turn, be deuterium replenished from one of these inexpensive feedstocks.
A process for deuterium replenishment of TFM has been demonstrated by Andreades; namely, the exchange of TFM and methanol-OD, catalyzed by the base sodium methoxide. The exchange rate is too slow, however, to be practical for the present purpose. (J. Am. Chem. Soc. 86, 2003 (1964)).